Initally called the Modding API, it has been planned in some capacity since at least July 5, 2010, shortly after the release of Alpha 1.0.1_01. It was then stated to be released in Beta 1.8.[6] The Modding API was then rebranded as the Plugin API, with the release originally stated to be planned for 1.3, then for 1.4, and then it was accidentally stated by Curse that it would be implemented in 1.5, but has since been delayed...At MineCon 2012, Mojang shared their vision for the future of the plugin API....As of October 26, 2013, the Plugin API/Workbench cannot be found on its GitHub page. Furthermore, the developer website has also been taken offline[1]
Draw your own conclusions.